The ability of the mammalian brain to undergo long-lasting, activity-dependent changes in synaptic function and structure importantly contributes to the neural circuit modifications that underlie many forms of adaptive and pathological experience-dependent plasticity, including learning and memory. A leading model for such synaptic plasticity is NMDA receptor-dependent long-term potentiation (LTP). Although progress has been made in understanding the mechanisms underlying LTP, much remains unknown. This proposal tests the novel hypothesis that postsynaptic complexins play a mandatory role in the trafficking of AMPA receptors to the plasma membrane during LTP and also in the growth of dendritic spines that accompanies LTP. Complexins are critical for the calcium-dependent regulation of transmitter release via direct interactions with the SNARE proteins that are required for presynaptic vesicle fusion. This project will perform experiments that for the first time examine the synaptic functions of postsynaptic complexin. The experiments use a molecular replacement strategy, which incorporates the simultaneous, viral-mediated expression of multiple shRNAs to knockdown complexin levels as well as express a replacement version of wildtype or mutant complexin in single hippocampal pyramidal cells in vivo or in vitro. Electrophysiological assays in acute hippocampal slices and cell biological assays in cultured neurons will be performed to determine the consequences of the complexin manipulations on basal synaptic responses, LTP, long-term depression and AMPA receptor exocytosis and endocytosis. The role of postsynaptic complexin on the spine growth that accompanies LTP will also be examined. These experiments will elucidate novel molecular mechanisms by which excitatory synapses in the mammalian brain are likely modified during various forms of experience-dependent plasticity, including learning and memory. The results will also open up new, innovative areas of research on the postsynaptic membrane trafficking underlying LTP. In addition, this research will provide information that is critical for the development of agents that modify synaptic transmission in ways that promote cognitive function and alleviate psychiatric symptoms.